Hello Scotty, I have a 2001 Yukon XL, 209k mi, auto, 5.3 Ltr. that started misfiring on all cylinders on cold start up. Code is P0300. Misfire only occurs for a few seconds on cold start up, shutter sound, and then returns to normal operating sound. It triggers the service light and the P0300 code-no other code triggered. Doesn’t happen again when engine is warm, only at cold start. Right before this I had received a code for the bank 2 downstream 02sensor and replaced it with an AC Delco sensor. Not sure if that is related. The Yukon runs great and I just returned from a 6,000 mi trip from CA to PA-even returning with a loaded trailer attached. Please help! Awesome channel!
Could be a leak(s) in your intake manifold gasket(s).
Heat expands the parts and the leak goes away after the engine warms up a little.
A leaking vacuum hose likely wouldn't stop leaking after the engine warms.
Maybe begin with an easy, cheap test. (the cost of a can of carb or brake cleaner)
On the first start of the day
hook up your scanner and switch to Live Data.
Observe your Short Term Fuel Trims (STFT) at cold idle while watching your Engine Coolant Temperature (ECT).
See if STFT's are high at startup, for either or both cylinder banks, and what the ECT is when they start going down. (you'll also confirm that your ECT sensor is working)
Knowing what the approx engine temp is when the problem "self corrects" will give you an idea of how much time you have to do the next test
On the first start of another day
Grab a can of carb cleaner, starting fluid, or my favorite flammable aerosol, brake cleaner.
Hook up your scanner, switch to Live Data, and get ready to watch your short term fuel trims.
Also, watch your engine coolant temperature. You now have an idea of how hot it gets before the problem "self corrects".
So start the engine, and look at the STFT values. Then spray the flammable aerosol of your choice all along the base of the intake manifold (where it mates to the cylinder head) of the cylinder bank that's showing high STFTs at cold-start.
If it's both banks,
Do bank 1 first. When you spray around the intake manifold's base does the STFT value come down on bank 1?
Then do bank 2. When you spray around the intake manifold's base does the STFT value come down on bank 2?
If it does, then it's time to replace the intake manifold gasket(s).
.
Update: It was the intake manifold gasket. Replaced with Felpro gasket and all good. Thanks for help!
Thanks for letting us know
